If you do not owe any taxes at the time of the extension, Form 4868 line 6 should be 0, but you are referring to the tax liability reported, line 5. That is not as important as line 6, ehich you entered correctly.
If you were to owe tax in the future,there are several ways the IRS accepts payment including installments:
https://ttlc.intuit.com/questions/1901676-how-can-i-pay-my-federal-taxesHere is a resource to see how much the penalties and interest could be if you did owe...it compounds daily. This link looks like it is not for individuals, but it is for individuals.
https://www.irs.gov/businesses/small-businesses-self-employed/understanding-penalties-and-interest
